100 LITERATURE ON TORRENTS. 



"All the places mentioned are covered with very good turf, and the 

 goil is formed of grey schist. 



"The canton of Tessino is destitute of wood in most parts, but, as 

 compensation, it possesses magnificent pastures which, in spite of the maddest 

 mode of depasturing, preserve (thanks to the wonderful soil) their thick and 

 perennial mantle of verdure. The inundations there are terrible, although 

 at the lower end of the torrents are to be found lakes which retain the 

 alluvial soil and moderate the rapidity of the rise in the rivers ; the effect 

 produced by these lakes is very great but insufficient ; in 1868 the level of 

 Lake Majeur rose 7 mfetres at Locarno, and in the public square the water 

 rose to the first storey of the houses. 



" It would be interesting and useful to ascertain the quantity of water 

 which, falling with the rapidity of a thunder shower, would be sufficient to 

 saturate a turfed surface, but the quantity is very small, and depends on 

 the steepness of the descent. ■ This phenomenon is easily explained. Alpine 

 turf, beat down by cattle, is formed of small plants growing close together, 

 the interlaced roots forming a sort of felt. When rain comes it makes the 

 rootlets swell, which, pressed together, imprison the soil and form a scarcely 

 permeable covering, through which the water gradually passes only by means 

 of capillarity and hydroscopicity. If the rain is slow and continuous these 

 two properties are enough to permit all the water to pass through. If, on 

 the contrary, it is violent, the water runs over the sijrface without being 

 absorbed. 



" But, supposing the surface to be horizontal, the effect just described is 

 also produced ; the excess of fallen rain, however, lies on the herbage to be 

 gradually absorbed, for the quantity of rain retained by the herbage is in 

 inverse proportion to the slope of the ground, and varies continually; 



" Turf, from the special point of view which we occupy, is chiefly useful 

 in consolidating the soil ; this end is partially attained, in so far as any 

 surface whatever, when turfed, will always resist the direct action of the 

 rain, but as a whole it is not : the excess of the water absorbed unites, 

 forms little streamlets, and, according as the inequalities of the ground on 

 which they recur are steep or narrow, the turf is attacked by the running 

 water, the soil is laid bare at some one point, and in a few minutes there is 

 the beginning of a ravine, which will always grow larger after every new 

 storm if a remedy be not promptly applied. 



" To return to the subject, I would say that turf increases the capil- 

 larity and hydroscopicity of the surface of the ground, but these two pro- 

 perties are unable to absorb spontaneously all the rain which falls durino- 

 storms, and the excess runs down the surface if the ground be steep, or 

 lies on the top if it be flat. 



" These observations lead me to conclude that all flat surfaces may be 

 turfed without inconvenience ; and that the turf on steep declivities will 

 moderate very slightly the rising of the floods. 



" There is plenty of opportunity for making experiments on this point. 

 Places for making observations should be chosen on the same kind of soil, 

 the surfaces of which have been examined, and where there is no water but from 

 the sky. It can be easily done in the mountains. So soon as there are signs 

 of a storm those employed should repair to such spots, there to measure the 

 quantity of rain with a pluviom^tre j and to measure, by means of barrages 

 with rectangular sections, the quantity of water issuing from the basin 

 where it is received ; and also to note the duration and nature of the storm 



